Edit: just realized that the above comment by wedrifid gives pretty much a summary of mine.
Scientists are handicapped in these debates, whatever their status. Their goal is to convince the debate's audience using scientific facts and logic. The goal of the creationists, however, is to convince the audience by whatever means work best.
So they will lie, invent and suppress facts, avoid or ignore questions, use ad hominem attacks, and generally use every kind of psychological or rhetorical tactic they can. And they're quite good at this, so that only trained rationalists might be completely immune. The only way to combat this is to use similar Black Arts. Scientists don't do this because they fear it would subvert the honesty of their science or that people who notice their behavior won't trust them in the future.
Science is a rational process. The fight against creationism (as against some other things) is fundamentally a fight against explicit and deliberate antirationality. When some creationists see a debate where the scientist clearly wins, it only serves to strengthen their creationist faith. Others are clear-thinking enough to be swayed by argument and we should do everything we can to expose them to it. In between are those who will abandon creationism once enough of their peers (or auth. figures, media persons...) do so. Do we have an estimate of how many are in each category?
So they will lie...And they're quite good at this
Are they quite good? where do you get this belief? quite good compared to what?
(EDIT: Woozle has an even better idea, which would apply to many debates in general if the true goal were seeking resolution and truth.)
Friends, Romans, non-Romans, lend me your ears. I have for you a modest proposal, in this question of whether we should publicly debate creationists, or freeze them out as unworthy of debate.
My fellow humans, I have two misgivings about this notion that there should not be a debate. My first misgiving is that - even though on this particular occasion scientific society is absolutely positively not wrong to dismiss creationism - this business of not having debates sounds like dangerous business to me. Science is sometimes wrong, you know, even if it is not wrong this time, and debating is part of the recovery process.
And my second misgiving is that, like it or not, the creationists are on the radio, in the town halls, and of course on the Web, and they are already talking to large audiences; and the idea that there is not going to be a debate about this, may be slightly naive.
"But," you cry, "when prestigious scientists lower themselves so far as to debate creationists, afterward the creationists smugly advertise that prestigious scientists are debating them!"
Ah, but who says that prestigious scientists are required to debate creationists?
Find some bright ambitious young college student working toward a biology degree, someone who's read Pharyngula and the talk.origins FAQ. Maybe have P. Z. Myers or someone run a test debate on them, to make sure they know how to answer all the standard lies and are generally good at debating and explaining. Then have the college student debate the creationists - if the creationists are still up for it. If not, of course, we can all make a big ruckus about how Michael Behe is afraid to debate a mere college student, and have the college student reply to all requests to debate Richard Dawkins or supply a scientific authority for the TV networks. And if Michael Behe manages to defeat the college student, then he can go on to debate a PhD, and if that doesn't work, Behe gets to talk to P. Z. Myers, and in the unlikely event Behe manages not to get his butt handed to him by P. Z. Myers, he would have earned the right to debate Richard Dawkins.
If we're dealing with young-earth creationists, then we add a bright 12-year-old at the start of the chain.
That way, anyone who wants to know the state of the debate and the status of the arguments, is welcome to watch creationists being beaten up by some college kid - armed with real science, mind!
But there will still be a debate. And if the scientific community, at some point in the future, manages to go astray on some issue where the opposing side seems "silly", then we can hope - if public debate is any use at all - that the challenger will gently defeat the 12-year-old, unravel the college student, score points against the PhD, and hold their own against senior scientists. There would still be a path to victory for worthy new ideas, and not a general license for a community to shut down all debate it thinks unworthy.
It's this notion of shutting down debate that I fear as dangerous; and it seems to me that you can get just the same strategic conservation of prestige, by endorsing the principle of debate, but sending out some bright college students to present the standard position. If the "controversy" as shown on CNN consists of some ID-er with a sober-looking business suit and an impressive-sounding title, versus a TA in jeans to represent the scientific community - but with accurate science, mind! - then I think this would viscerally answer what the scientific community thinks of creationism, and not create the false impression of an ongoing debate, while still giving airtime to the standard scientific replies. If CNN isn't interested in showing that "controversy" - well then, that tells us what CNN really wanted, doesn't it.
If an idea is so completely ridiculous as to be unworthy even of debate - then send out some bright un-titled college students to debate it! Do vet them for knowledge of standard replies, explanation ability, and debating ability against evil opponents, to make sure standard science is not needlessly embarrassed. But there should be plenty of ambitious young bright college students who can pass that filter and who would enjoy some TV exposure.